As is well known, a baler is an agricultural machine that picks up crop from the field and gathers it into bales, which can be either rectangular or round. In the case of a square baler, the bales are made in a baling chamber that has a rectangular cross section. The crop picked up from the ground is driven into a feed duct. When the feed duct is full, its contents are partly compressed and pushed upwards by a paddle into the baling chamber. The baling chamber has a reciprocating plunger that compresses this “slice” of crop by pushing it deeper into the baling chamber and the process is repeated for successive slices until the bale has the desired length. At this point, loops of twine are tied around the crop to keep it as a compressed bale. This bale is discharged onto the ground by the next bale that is being formed in the baling chamber, the latter bale pushing against the previously formed and finished bale.
Rectangular bales conventionally are able to maintain their shape because of a series of parallel extending twine loops which are provided lengthways around the bales. Agricultural balers have been available for many years which use automatic knotters that serve to tie one or two conventional knots on every loop for binding a bale. When a bale reaches its desired length, a knotting cycle is started. During this cycle, in the case of a double knotter, two knots are formed, the first knot for closing the loop of the finished bale and the second knot for starting the loop of the next bale. The construction of such balers is well known and is fully documented in numerous prior art references. For this reason, a more detailed explanation of the baler is not deemed to be necessary within the present context.
The construction of the knotters is also well known but will be described herein briefly to explain the background of the invention. The main part of the knotter is a billhook that engages the twines to be tied together while they are held under tension. The billhook is rotated through 360° about an axis perpendicular to the twines and in the process wraps a loop of the twines around itself. The twines are then cut and the free end of the twines thus created is pulled through the loop to complete the knot. For a fuller explanation of the operation of the knotter, reference may be made for example to U.S. Pat. No. 4,142,746 which is here incorporated by reference.
It will be noted that, during the formation of the second knot, the billhook needs to operate on a length of twine that is maintained taut. To achieve this, the twine on one side of the billhook is held firmly while on the other the two runs of the twine from the supply rolls are each tensioned by a friction clamp or another tensioning means, such as tensioning rollers, and a spring biased tensioning arm that takes up slack in the twine. A simple clamp can be found in U.S. Pat. No. 4,074,623. Rollers are shown in U.S. Pat. No. 4,196,661.
Two tensioning devices are required for the two runs, which are referred to as the upper and lower tensioning devices. The operation of the lower tensioning device is more critical than the operation of the upper tensioning device, because it is disposed at a greater distance from the knotter, so that there is a greater length of twine which must be jerked away during the final stage of the knotting process. The lower tensioning device thus has to act more vigorously in order to compensate for the stretch of the twine length between the tensioning device and the knotter.
FIGS. 1 and 2 of the accompanying drawings show the construction of a known tensioning device. The tensioning device has a tensioning arm 10 which is shown in FIG. 1 in its raised position and in FIG. 2 in its lower position. The tensioning arm 10 is mounted on a fixed bracket 12 for pivotal movement about a pin 23. At one end, the arm 10 is acted upon by a spring 14 which biases it in a direction to take up slack in the twine 16, which passes around a roller 18 at the free end of the tensioning arm 10. The run 16a of the twine 16 leads to the knotter and the other run 16b is drawn from a supply roll (not shown) through a guide ring 32 and a clamp 20. Within the clamp 20, the twine 16 passes between two cogs 22 and 24, of which the cog 22 is mounted on the stationary bracket 12 and the other is carried by a U-shaped brace 26. At one end, the brace 26 is free to pivot about a support pin 28 while a pair of springs 30 (only one shown), adjustable by means of nuts 34, act on the opposite end of the brace 26 to urge the two cogs towards one another.
The clamp 20 and the tensioning arm 10 maintain the twine under tension both during the formation of a bale and during the tying of a knot. The tension, which is usually adjustable, is set by the nuts 34 acting on the springs 30 of the clamp 20. For good operation, the force applied to the twine by the clamp 20 needs to be greater than the force applied by the tensioning arm 10. The basic function of this arm 10 is to take up the slack twine during and after formation of the second knot. The arm needs to have enough travel to stretch all the slack twine and still to be able to pull the second knot tight after its ejection from the bill hook.
The setting of the twine tension by adjustment of the clamping force on the twine is necessarily a compromise. If the twine is not sufficiently taut, the knots will not be formed properly. Furthermore, if the force of the tensioning arms is greater than the load applied by the clamp or rollers, this will result in there remaining insufficient arm travel for tightening the second knot. Otherwise, too high a tension in the twines causes wear, generates a great deal of noise and increases the risk of the knotter malfunctioning.
It can thus be understood from the above description that the clamp and the tensioning arms are two distinct elements each having their own settings, but nevertheless working closely in conjunction with one another.